Hydraulic cements



HYDRAULIC CEMENTS I Rufus V. Jones, Bartlesville, Okla assignmtoPhillips Petroleum Company, a corporation of Delaware 19 Claims. (Cl.166-31) This invention relates to cements having rm of hydration or set,to slurries of such cement, and to e me 0 making these slurries. Thecement with which the invention is concerned is preferably a Portland orPortland-type cement. In another aspect it relates to any hydrauliccement composition in a or with added water in an aqueous slurry form,which when in the form of an aqueous slurry has a retarded initial setor extended or retarded thickening time and/or a. reduced water-loss toadjacent porous formations, due 5 the addition of a minor but effectiveamount of acid $9511.25 sulfate sa 0 sm act stare su fate, thisinvention relating first to said compositions of matter, second toprocesses of compounding said compositions, and third to processes forusing said compositions in the arts of cementing wells, sealing porousformations during the drilling of wells, cementing casings in the well,squeeze cementing, plugging the well or the earth formation adjacent thesame, and grouting or sealing crevices, cracks or holes in man-madeformations, such as buildings, foundations, dams, breakwaters orconcrete and masonry structures, in some instances the cracks orfractures already existing before the slurry is pumped into them, and insome cases the pressure of the slurry being pumped into or against thesurface of said formation or structure forming by its pressure thecracks or fractures to be filled.

Two possible methods of making acid starch sulfate and its salts aredisclosed in my prior U.S. Patents 2,686,779 of August 17, 1954, and2,697,093 of December 14, 1954, filed July 18, 1949, and December 16,1949, respectively.

Among the objects of the invention is the provision of a cement having aretarded rate of hydration, or retarded set, as it will be hereinaftertermed, particularly at elevated temperatures up to and above 300 F.and/or at high pressures up to and above 20,000 pounds per square inch,such as are encountered in cementing of deep wells.

One object of the present invention is to provide a suitable hydrauliccement aqueous slurry, and suitable processes employing the same, forcementing casing in wells, for squeeze cementing in wells, and forgrouting cracks, fractures or voids, in natural formations, such as inwells, or in man-made formations such as dams, breakwaters, wells andmassive foundations and structures of all types.

Another object of this invention is to provide a dry hydraulic cementpowder which is a novel composition of matter, and which may be mixedwith water to form an aqueous cement slurry which is a novel compositionof matter and which has at least one of the following use- 65 fulproperties: a relatively retarded time of initial set, a relativelyextended thickening time during which it is pumpable, and/or a r lowwater-loss to mans formations with which it may come in contact duringcementing or grouting operatic "Ffith'e'r oB ects of file inventionreside in the provision es'MuMCROSS REFERENCE 5 as 4 nus/s Z2 ice of aslurry of the above cement, and in a method of making such slurry.

These and further objects of the invention will be more readily apparentin the following description.

In the cementing of oil wells it is customary to mix a hydraulic cement,for example a Portland or Portlandtype cement, with the requisite amountof water to form a pumpable neat slurry, and to pump the mixture intothe well and down the hole into the place where it is desired to have itharden. In present oil well drilling practice, with wells commonlyranging from 6,000 to 12,000 feet or more in depth, high temperaturesare encountered at the locations which are to be cemented, andrelatively long periods of time are often required to pump the slurryinto place. Furthermore, in the customary practice of pumping the cementslurry down through the casing and either forcing it out the bottom ofthe casing and upward around the outer surface of the casing, or throughperforations in the lower end of the easing into the formation sought tobe sealed, the slurry is required to pass through narrow channels andsmall openings. Successful placement of the slurry, therefore, requiresthat the slurry shall remain fluid and pumpable at high temperatures forseveral hours before it begins to harden. However, after the slurry hasbeen pumped into place, it is desirable to have the hydration or setproceed at a rate at which the slurry will attain its final set anddevelop considerable strength within a reasonable time, say within a fewdays. It would be even more desirable to have it attain its final set inabout 24 hours but often this is not attainable.

As pointed out in the preceding paragraph, the most important functionof the hydraulic cement aqueous slurry of the present invention is thatit has a retarded time of initial set, and therefore remains pumpablefor a relatively long period of time and a relatively long period oftime passes before it thickens, yet it will attain a final set of someconsiderable strength within a reasonable length of time so that thewell-drilling crew is not unduly delayed, but can get back to work andproceed to continue drilling the well bore, or to perforate the casingand/or cement with the usual gun perforating tools known when the acidstarch sulfate or salt of starch sulfate is carefully prepared so that arelatively high degree of sulfate substitution has occurred withrelatively low amounts of degradation of the starch molecules, asecondary effect is achieved, which, while not as important as the firstmentioned etfect of delaying the time of initial set and extending thethickening time of the cement, is also of considerable value incementing oil wells, namely, the aqueous cement slurry containing theminor but effective amount of acid starch sulfate or salt of acid starchsulfate has a reduced tendency to lose water to porous formations acrossthe surface of which it must pass in going to its intended position inthe well. Many failures in prior art oil well cementing jobs, which havebeen accredited to the premature setting of the cement, are thought tobe caused actually by the formation dehydrating the cement slurry,thereby rendering the cement immobile before it reaches the desiredposition. As the practice of using scrapers to clean the mud off thewell walls to obtain a better cement-formation bond becomes morefrequently used, the better the formations will absorb water from thecement slurry causing it not only to plug the annulus between the casingand the wall of the 0 well, but also to have insufiicient water fornormal by dration upon setting, and the greater will become therealization of the need for low water-loss cements.

j ggunanmeutg ywasnimbe, gen; rn rcta y in the practiceTif the presentinvention, and

Everything which is said applying to natural formations in wells appliesalso in some degree to man-made formations being grouted, and the wordformation as used herein is regarded as generic to natural earthformations, geological formations, and man-made formations such asstructures.

In the prior art of squeeze cementing in wells and in forcing grout intothe cracks and crevices in fractured foundations or the like, it hasbeen the practice to employ as a breakdown agent, water or drilling mud,which is forced ahead of the aqueous hydraulic cement slurry into theformation to split the same and enlarge the fractures or cracks to befilled, because if ordinary hydraulic cement aqueous slurry wereemployed it would lose water to the formation or foundation so rapidlythat the cement slurry would start to set before much penetration hasbeen effected. When a relatively low water-loss hydraulic cement aqueousslurry is employed, the amount of breakdown agent can be greatlyreduced, or entirely eliminated, because the low water-loss cementslurry will penetrate to much greater distances before losing sufficientwater to be caused to set by this dehydration. When squeeze cementing inoil wells is involved, in which it is desired to force a thin disk orlayer of these cement slurries out into a natural earth formation alongpre-existing or pressure made fractures, in order to separate an oilsand from some other sand at the general vicinity where the oil wellintersects the same, it is especially advantageous to use a relativelylow water-loss cement slurry as breakdown agent because then less wateris likely to be absorbed by the oil formation where it might cause areduction in the present or future amount of production of oil. Someoil-bearing formations contain bentonitic materials which swell whenthey encounter water, and if excess water is injected into suchformations, the swelling of the bentonitic material may prevent futureproduction of oil.

It is not believed necessary to have a drawing, or to describe minutelythe well known cementing operations disclosed in such patents as US.2,795,507 and 8.

By hydraulic cement this invention intends to include all mixtures oflime, silica, and alumina, or of lime and magnesia, silica and aluminaand iron oxide (magnesia for example may replace part of the lime, andiron oxide a part of the alumina), as are commonly known as hydrauliccements. Hydraulic cements include hydraulic limes, grappier cements,puzzolan cements, natural cements and Portland cements. Puzzolan cementsinclude slag cements made rom slaked lime and granulated blast furnaceslag. Because of its superior strength Portland cement is preferredamong the hydraulic cements, but as the art of cements recognizeshydraulic cements as a definite class, and as results of value may beobtained with acid starch sulfate, or salts of acid starch sulfate, withany member of that class, it is desired to claim all hydraulic cements.In addition to the ordinary construction grades of Portland cement orother hydraulic cements, modified hydraulic cements and Portland cementsdesignated as high-early strength cement, heat resistant cement, andslow-setting cement may be used in the present invention.

In most oil well cementing and grouting operations it is generallydesirable to use neat cement for added strength, but obviously it isalways possible to add to the hydraulic cement, water, and acid starchsulfate or salt of the same, any desired amount of an inert granularfilling material or aggregate such as sand, ground limestone, or any ofthe other well known inert or even cementitious aggregates, as long assimple tests show the amount added does not reduce the compressivestrength after final set below the desired value. For example, inplugging porous formations, bentonite or other clays are often added tohydraulic cement aqueous slurries, as taught by US. Patent 2,041,086 ofMay 19, 1936, or iron oxide or barium sulfate is added to make heavycement. Any of these aggregates can be added to the aqueous hydrauliccement slurry of the present invention in the usual proportions used inthe prior art.

In operations in previously uncased wells it is often desirable to useneat cement in the practice of the present invention, because inertfilling material may automatically become detached from the walls of thewell, and will tend to mix with and dilute the slurry to such an extentthat it is undesirable to add any filling material to the slurry beingforced into the well. It is customary in the prior art when cementing tomake simple tests as to time of set, compressive strength, etc., onsamples of the proposed mix.

The amount of water added to the cement of the present invention is notcritical, it being obvious that sufficient water should a pumpable 51mth en the slurry becomes pump er water need be added. One advantage ofthe slurry of the present invention when a relatively less degeneratedacid starch sulfate or salt of the same is used is that it is low w andtherefore it is not necessary to add much excess water over the amountmaking the slurry pumpable as a reserve for expected losses, whichexcess water might tend to reduce the final compressive strength of thecement.

It has been found that all hydraulic cements, especially Portland andPortland-type cement aqueous slurries can be retarded in setting time,the time of thickening extended, and in some cases the water-losstendencies retarded, so that they meet all the above requirements forthe satisfactory cementing of deep wells and like operations by theaddition of a minor but elfective amount of fro 0.05 to 5% b w ht of thedry hydraulic cement of acid starch sulfate, or the metal, ammonium ororanic O- without seriously affecting the other desira e properties ofthe cement. It is preferred at present to use the sodium or potassiumsalts of acid starch sulfate merely because these salts are readilyavailable commercially and therefore relatively inexpensive. However,good results will be obtained using any other alkali metal salt, such asthe lithium, rubidium, caesium and other rare alkali metal salts, or theammonium or organic base salts of acid starch sulfate, all of which arewater soluble. Typical organic base salts that can be used are thosederived from ammonia such as methyl amine, dimethyl amine and quaternaryammonium bases; also pyridine, morpholine, and the like. In addition thealkaline earth metal salts such as the barium, calcium, strontium, andmagnesium, and the heavy metal salts such as the aluminum, iron, copper,lead, silver, mercury, nickel, and all other salts of acid starchsulfate (which are probably insoluble in water but which hydrolyze inthe hydraulic cement aqueous slurry which is an aqueous alkalinesolution) are just as useful in this invention in the aqueous hydrauliccement slurry which is quite alkaline. Acid starch sulfate and all ofits salts, whether such salt is formed in the aqueous hydraulic cementslurry by hydrolysis of some water-insoluble salt, are all valuable inamounts of 5 percent or less, based on weight of dry cement, inretarding the set of aqueous hydraulic cement slurry, especially at thetemperature and pressure encountered in cementing a well, and in manyinstances the acid starch sulfate or salt will decrease the water lossfrom said aqueous hydraulic cement slurry to porous formationsencountered in the well.

While 0.05% to 5% of acid starch sulfate or its salts by weight of thedry hydraulic cement will give valuable results, it has been found thatfrom 0.2 to 1% is the most preferred range in wells less than 14,000feet deep and less than 300 F., the use of 0.5% being particularlyeffective in such wells, and the percentage above 1% being chiefly ofvalue in still deeper and hotter wells.

The term acid starch sulfate and its salts includes all starch compoundswhich may be regarded structurally as being the starch monoesters ofsulfuric acid and themetal, ammonium or other salts thereof. The starchacid sulfate esters are characterized by the typical sulfate linkage:

wherein S is sulfur, and O is oxygen.

It will be seen that the reaction product of starch and sulfuric acid orchlorosulfonic acid contains the sulfate linkage:

OH 011 OR X401?! +nClS-OH i(O- OH +nHCl OH i wherein is ananhydroglucose unit of the starch molecule and n is the number of suchunits in such starch molecule, the formula showing mono-substitution ineach unit, whereas in actual practice some units have double or triplesubstitution and many units have no substitution at all, the degree ofsubstitution merely being an average value.

Acid starch sulfate and its salts may be prepared by a number ofsulfating methods old in the prior art, employing either sulfur trioxidegas and the starch alone, or sulfuric acid and/or sulfur trioxide alongwith various diluents to retard the reaction making it a sulfationreaction with the starch, and preventing oxidation, and no matter howthe acid starch sulfate is made, it and any of its salts will act toretard the set of a hydraulic cement aqueous slurry and to increase thetime before the cement thickens to the point where it is not pumpable.While not to be considered as limits, a relatively high degree ofsulfate substitution for the present invention is about 0.2 or more outof the 3.0 possible in each anhydroglucose unit in the starch molecule(which unit has 3 hydroxyl groups which can be substituted).

The acid starch sulfate or salts thereof may be prepared by any one ofseveral methods as described above, but one of the preferred methods isthe reaction of starch with a complex of sulfur trioxide, for example,pyridine sulfur trioxide, in the presence of a tertiary amine, such aspyridine, and an inert diluent to form the desired product. The specificdetails of the process described in the last sentence are outlined in mysaid Patent 2,697,093, in which said process of preparing acid starchsulfate and its salts is claimed. The acid starch sulfate may also beprepared by the reaction of starch with an alkali metal chloro-sulfonateunder anhydrous conditions, and in the presence of an inert diluent suchas benzene, and a tertiary amine. The specific details of this processare outlined in my said Patent 2,686,779, in which this process ofproducing acid starch sulfate and its salts is claimed. Still othermethods of preparing acid starch sulfate and salts of the same includethe reaction of starch with chloro-sulfonic acid in the presence of aninert diluent and tertiary amine.

Portland cement is a mixture of complex silicates and aluminates ofcalcium containing excess lime. The setting or hardening is a result ofthe hydration or other chemical readjustments of the various components.Generally speaking. three periods in the set are recognized: initial,final and hardening sets. The initial set normally occurs at ordinarytemperatures in from one or two hours after the mixing, the final settwo to five hours later and the hardening continues for an indefinitetime but it is substantially complete in about 30 days.

The initial set is said to have occurred when a cement slurry has lostits plasticity to such a degree that the two pieces of a broken specimenwill not unite to form a homogeneous mass when placed in close contact.The individual grains of a cement slurry must remain undisturbed inintimate contact with each other for a time before the initial setoccurs in order to produce a coherent mass. Agitation during the latterpart of the period of initial set will prevent the cement from hardeningproperly to the desired homogeneous, coherent mass.

In order to form a perfect seal in cementing wells, it is necessary thatthe cement be placed before the initial set occurs and it is desirablethat it be placed and allowed to stand for a short period before theinitial set begins. With the equipment available, there is a limit tothe time in which it is possible to mix a cement and pump it into thebottom of the well and up around the casing to the location desired.

Another reason it is necessary to have the cement in place before theinitial set begins is that the viscosity rises as the settingprogresses. This increases the difiiculty of pumping and is undesirablebecause of the added strain on the pumping equipment.

It is possible to retard the rate of set, within narrow limits, byincreasing the alumina content of the cement, but this method is notwidely used because of the high cost of high alumina cements and thelimited effective range. The rate of set can be retarded also byincreasing the amount of water present in the mix. However, above about35 to 50 percent water, based on the weight of dry cement, increasedamounts of water will result in weaker cement and there is no way ofknowing exactly how much dilution will result from water encountered inthe well. Addition of small amounts of gypsum, or calcium sulfate willresult in a retarded rate of set, but an excess will increase the rateand may cause the cement to disintegrate or be weakened. It istherefore, highly desirable that a retarded cement such as mine beavailable for cementing work.

The most convenient method of using acid starch sulfate or its salts incement is to run the same and the hydraulic cement through a rotarymixer to produce intimate mixing and later add water to form a fluidslurry. However, the acid starch sulfate or its salts may be addeddirectly to the cement and water at the time of mixing at the well, orthe acid starch sulfate or its salts may be dissolved in the water withwhich the cement is mixed, with substantially the same result. Themethod of mixing is not critical as long as a somewhat uniform mixtureis produced.

The rate of hydration or set of cement is ordinarily increased by anincrease in temperature. Since the bottom hole temperature in the wellmay be considerably higher than the atmospheric temperature, it isdesirable that a method such as I have described be available for use inthe cementing of oil wells. My method is effective at elevatedtemperatures as well as at ordinary atmospheric temperatures, becauseobviously a set retarding agent operative at atmospheric temperatureswill also retard the set at higher temperatures.

While it is not desired to limit the present invention by any theory ofoperation and while the scope and validity of the claims do not dependupon the validity of any theory of operation, it is believed helpful inunderstanding the invention to think of the acid starch sulfate or itssalts temporarily absorbing so much of the water that the Portlandcement is only slowly able to obtain enough water to make its initialset, whereby the initial set of the cement is greatly retarded. Finallythe Portland cement particles take the water away from the water solublestarch particles and attain an initial and then a final set withsuitable strength in the cement for use in oil well cementingoperations.

EXAMPLE I A neat Portland cement aqueous slurry having a density of 14pounds per gallon was tested without any additive,

and with 1% of the weight of the dry cement of sodium starch sulfate(abbreviated Na starch S in Table I below). To furnish a comparison,mixtures of a similar 1% of ordinary soluble starch and two trade markedsoluble starches (Shopal 8-A and Amioca) in the same slurry are alsoreported.

was decanted and the solid reaction mass warmed with methanol. Lumpswere broken up by treatment in a Waring blender after which the productwas washed free from sulfate ions with methanol.

Pyridine starch sulfate thus prepared was used to prepare the acidstarch sulfate and the desired salts.

Table I Percent Percent Halli- Water-l mlsJmrns' water addition bnrtonTest No. Cement as rcent. Additive as percent thickeneeceing time 3 min.at 1 hr. at

ment ment at 180 F. 75 F. 180 F.

1 Portland. 40 0 0.8 4.5/0.4 '2 do 40 1.0 18/0.3 18I0.3 a (in 40 1.01110.4 Set 4 (10.-.. 40 Amioca.. 1.0 1610.1 Set 5 n 40 Na starch S0 1. 0/1! These slurries were all tested for water-loss according 20 Thesodium starch sulfate was prepared by making a to the procedure setforth in API Code 29 for drilling muds. This test measures the cubiccentimeters of filtrate that can be forced through a standard filterpaper in 30 minutes by a 100 pounds per square inch gauge pressuredifferential, which has been found to give a very good indication ofwhat would be lost to an exposed porous sand formation in an oil well,whether from the well drilling mud or from cement being placed in thewell.

Obviously the sodium starch sulfate ester is much more effective thansoluble starch in reducing the excessive water loss of the Portlandcement slurry alone. The neat Portland cement slurry lost ml. of itsavailable water in 0.4 minute, becoming immediately unpumpable, and theslurries with the soluble starches reached the same unpumpable state inabout the same time, but that with the sodium starch sulfate was just aspumpablc at the end of 20 minutes of exposure to water-loss as it was atthe start, except that it was 20 mintues further along in the process oftaking an initial set. This would give a tremendous advantage in placingcement in back of casing in a well where it had to be pumped upwardsthrough a narrow annular space past a considerable area of exposedporous sand or rock formations, and would insure placing the cement (bypumping) much further up along the outside of the casing than couldotherwise be done, resulting in sealing ofi more extensive formationsand making a better and more extensive bond between the casing and thewell hole formations than ever possible before.

EXAMPLE H The substantial equivalence of the various salts of acidstarch sulfate is demonstrated by the following tests in Table II. Arepresentative hydraulic cement was mixed with water to form an aqueousslurry weighing 14 pounds per gallon and tested with and without theadditives listed at 180 F. to show how high temperatures in a well wouldaffect it, while the time of thickening was tested according to API Code32 to determine how long it took the slurry to attain a viscosity of 100poises as it set, this figure being chosen because slurry over 100poises viscosity is getting diflicult to pump into a well, and of coursethe compressive strength in pounds per square inch was measured at 180F. at 1 and 7 days after it set, all as reported in Table H.

One method of preparing the various acid starch sulfates and acid starchsulfates salts for these tests will now be described. Two hundred gramsof Amioca starch, a trade marked product consisting of starch from awaxey maize, 611 grams of pyridine-S0 complex and 1800 grams each ofpyridine and benzene were heated and stirred for 8 hours at a refluxtemperature of 194 to 196 F. After two hours the reaction mixture becamepasty. The starch sulfate was recovered as the pyridine salt. Afterstanding overnight, the clear solvent layer methyl alcohol slurry ofsome of said pyridine starch sulfate and adding an excess of sodiumhydroxide in methyl alcohol and stirring the same together for one hour.The mixture was filtered, washed with methyl alcohol, and dried in avacuum desiccator. Sufiicient sodium starch sulfate was prepared to makethe ammonium and acid starch sulfates. Some of the sodium starch sulfateprepared as above was used to make a 10 percent aqueous solution of thesame. This was stirred vigorously 30 minutes and then passed through acation exchange resin to form the acid starch sulfate. Afterward some ofthe resulting acid solution was neutralized with ammonium hydroxide togive ammonium starch sulfate. The calcium starch sulfate was prepared bymaking a methyl alcohol slurry of the pyridine starch sulfate andstirring the same with an excess of calcium chloride in a methyl alcoholsolution for one hour. The product was filtered, washed free of chlorideions, and dried in a vacuum desiccator.

Other processes have been used to prepare these same salts, and thesalts so prepared had substantially the same treating value in aqueouscement slurries, so the mode of preparation is unimportant provided thechemical product desired is produced.

Table II.Tests of Example I! Amount Thlck- Compressive add. suingstrength (pertime, cent hnzmin. Test Additive by to reach P.s.l. P.s.i.No. weight after 1 after 7 of dry lses day at days at cement) M1180" 180F. 180 F.

N one 0 1:25 2, 740 2, 850 Acid starch sulfate 0.5 4:50 1,860 3,260Sodium starch sulfate 0. 5 3:45 1, 1,410 Calcium starch sulfate 0. 55:39 380 583 13--- Ammonium starch sulfate..- 0. 5 3:25 2, 860 2, 580

Table 11 clearly shows the increase of thickening time under deep welltemperature conditions given by 0.5% by weight of the dry cement of acidstarch sulfate and various salts thereof to an aqueous hydraulic cementslurry, all with the retention of suificient compressive strength to beuseful in well cementing.

The retarded set, or extended thickening time, was best measured by theHalliburton thickening time by which is meant the time at which thesetting cement slurry reaches a calibrated 100 poises of viscosity,which viscosity is approaching about the limit in increasing viscositythat is readily handled by pumps through some 9 thousands of feet ofcasing and well bore outside the casing in a well.

Baroid filter presses operated at 100 pounds per square inch were usedto determine the water-losses of cement slurries.

Thickening times of cement slurries were measured at atmosphericpressure with a Halliburton consistometer.

The water-losses of the cement slurries were determined by the procedurespecified for use on drilling fluids by API Code 29 (1942). Thetreatment of the cement slurry preceding the water-loss determinationconsisted of mixing the water and cement for three minutes to form theslurry and then storing the slurry in sealed jars in an oven at 180 F.

Thickening times of cement slurries were measured at atmosphericpressure in a Halliburton consistometer according to the proceduredescribed in API Code 32, section XII, paragraphs 54 and 67 (1947).Throughout this report thickening time refers to a Halliburtonconsistometer (atmospheric pressure) thickening time unless otherwisespecified.

While numerous examples of the invention have been given for purposes ofillustration, the invention is not limited thereto.

Having described my invention, I claim:

1. A cement capable of forming a fluid slurry when mixed with water,said cement having an extended thickening time, said cement comprising ahydraulic cement mixed with a minor proportion by weight of the drycement of a hydraulic cement thickening time extending agent selectedfrom the group consisting of acid starch sulfate and salts of acidstarch sulfate.

2. A cement capable of forming a fluid slurry when mixed with water,said cement having an extended thickening time, said cement comprisingPortland cement mixed with a minor proportion by weight of the drycement of a hydraulic cement thickening time extending agent selectedfrom the group consisting of acid starch sulfate and salts of acidstarch sulfate.

3. A cement capable of forming a fluid slurry when mixed with water,said cement having an extended thickening time, said cement comprising ahydraulic cement mixed with 0.05 to by weight of the dry cement of ahydraulic cement thickening time extending agent selected from the groupconsisting of acid starch sulfate and salts of acid starch sulfate.

4. A cement capable of forming a fluid slurry when mixed with water,said cement having an extended thickening time, said cement comprisingPortland cement mixed with 0.2% to 1% by weight of the dry cement ofalkali metal starch sulfate.

5. A hydraulic cement slurry having a retarded setting time attemperatures above atmospheric, comprising a hydraulic cement, water anda minor proportion by weight of the dry cement of a hydraulic cementthickening time extending agent selected from the group consisting ofacid starch sulfate and salts of acid starch sulfate.

6. A hydraulic cement slurry having a retarded setting time attemperatures above atmospheric, comprising Portland cement, water and aminor proportion by weight of the dry cement of alkali metal starchsulfate.

7. A cement capable of forming a fluid slurry when mixed with water,said cement comprising a hydraulic cement mixed with a minor proportionby weight of the dry cement of sodium starch sulfate.

8. A cement capable of forming a fluid slurry when mixed with water,said cement comprising Portland cement mixed with a minor proportion byweight of the dry cement of alkali metal starch sulfate.

9. A hydraulic cement slurry comprising a hydraulic cement, water, and aminor proportion by weight of the dry cement of alkali metal starchsulfate.

10. A hydraulic cement slurry comprising Portland cement, water, and aminor proportion by weight of the dry cement of sodium starch sulfate.

11. In the method of cementing a casing in a well which comprisespumping down through the casing and upwardly in the annular spacebetween the casing and the borehole an aqueous hydraulic cement slurry,the step of adding to the cement slurry a hydraulic cement thickeningtime extending agent selected from the group consisting of acid starchsulfate and salts of acid starch sulfate.

12. In the method of cementing a casing in a well which comprisespumping down through the casing and upwardly in the annular spacebetween the casing and the borehole an aqueous hydraulic cement slurry,the step of adding to the cement slurry alkali metal starch sulfate inamounts ranging between 0.2% and 1% by weight of the dry cement in theslurry.

13. In the method of cementing a casing in a well which comprisespumping down through the casing and upwardly in the annular spacebetween the casing and the borehole an aqueous Portland cement slurry,the step of adding to the cement slurry a minor but effective amount ofa hydraulic cement thickening time extending agent selected from thegroup consisting of acid starch sulfate and salts of acid starch sulfatesuflicient to extend the thickening time of said slurry.

14. In the method of cementing a casing in a well which comprisespumping down through the casing and upwardly in the annular spacebetween the casing and the borehole an aqueous Portland cement slurry,the step of adding to the cement slurry a minor but effective amount ofalkali metal starch sulfate sufficient to extend the thickening time ofsaid slurry.

15. In the method of cementing a casing in a well which comprisespumping down through the casing and upwardly in the annular spacebetween the casing and the borehole an aqueous Portland cement slurry,the step of adding to the cement slurry a minor but effective amount ofsodium starch sulfate sufiicient to extend the thickening time of saidslurry.

16. The process of producing a hydraulic cement aqueous slurry having anextended time of set which comprises admixing with hydraulic cement from0.05% to 5% by weight of the dry cement of a hydraulic cement thickeningtime extending agent selected from the group consisting of acid starchsulfate and salts of acid starch sulfate, and mixing and reactingtherewith sufiicient water to produce a fluid slurry.

17. The process of producing a Portland cement aqueous slurry having anextended thickening time which comprises admixing with Portland cement aminor proportion by weight of the dry cement effective to reduce saidwater-loss of said slurry of alkali metal starch sulfate and mixing andreacting therewith sufficient water to produce a fluid slurry.

18. The process of cementing a hole which extends into a formation whichcomprises placing a hydraulic cement aqueous slurry having an extendedthickening time adjacent to said formation by admixing with hydrauliccement from 0.05% to 5% by weight of the dry cement of alkali metalstarch sulfate, mixing therewith sufficient water to produce a fluidslurry and introducing said slurry into said hole into contact with saidformation.

19. The process of cementing a well which extends into a porousformation which comprises placing a Portland cement aqueous slurryhaving a reduced water-loss adjacent to said porous formation byadmixing with hydraulic cement a minor proportion by weight of the drycement effective to reduce the water-loss of said slurry of hydrauliccement thickening time extending agent selected from the groupconsisting of acid starch sulfate and salts of acid starch sulfate,mixing therewith sufficient water to produce a fluid slurry andintroducing said slurry into said well into contact with said porousformation.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Mathers Nov. 16, 1926 Swayze Apr. 24, 19455 Chapman May 7, 1946 Cannon Mar. 11, 1947 12 Larsen Mar. 11, 1947 AndesOct. 21, 1947 Ludwig Nov. 29, 1949 Ryan July 24, 1951 Lea July 24, 1951Ludwig Dec. 4, 1951 Clark Dec. 15, 1953

11. IN THE METHOD OF CEMENTING A CASING IN A WELL WHICH COMPRISESPUMPING DOWN THROUGH THE CASING AND UPWARDLY IN THE ANNULAR SPACEBETWEEN THE CASING AND THE BOREHOLE AN AQUEOUS HYDRAULIC CEMENT SLURRY,THE STEP OF ADDING TO THE CEMENT SLURRY A HYDRAULIC CEMENT THICKENINGTIME EXTENDING AGENT SELECTED FROM THE GROUP CONSISTING OF ACID STARCHSULFATE AND SALTS OF ACID STARCH SULFATE.